Computer viruses are a bad thing, but bacteria in your computer could be a good thing in the future. Researchers at the Chinese University of Hong Kong have demonstrated the ability to store data that is encrypted using bacteria. The team used a colony of E.coli for the demonstration.
The researchers were able to store enough data to put the Declaration of Independence inside 18 bacterial cells. When scaled up that would mean 1g of bacteria could store a whopping 90GB of data. The data can be encrypted by using site-specific genetic recombination.
The technique is promising, but is far from ready to be used inside your computer. Data retrieval requires a sequencer and is described as tedious and expensive. Another fear is that the bacteria might mutate over time and destroy some of the data stored inside.